Molecular Dynamics Studies of the Structural, Thermo-Mechanical and Finite Size Elastic Properties of Hexagonal Boron Nitride

Abstract

This thesis investigates the structural, thermo-mechanical and finite size elastic properties of hexagonal boron nitride (h-BN) using classical molecular dynamics simulations. At high temperatures, specific heat shows considerable increase beyond the Dulong-Petit limit which is interpreted as a signature of strong anharmonicity present in h-BN. Analysis of the height fluctuations shows that the bending rigidity and variance of height fluctuations are strongly temperature dependent and this is explained using the continuum theory of membranes. The observed Young’s modulus and Poisson ratio of h-BN is increase with system size in accordance with a power law, and are found to be anisotropic for finite sheets whereas they are isotropic for infinite sheets and they also satisfy Born’s criterion for mechanical stability. Using the formula derived from Foppl-von Karman plate theory, variation of bending rigidity with system size is determined from the measured value of thin shell thickness. As the system size increases, the zero Kelvin Young’s modulus also increases, which leads to an increase in the longitudinal and shear wave velocities. The strain fluctuation method is employed to investigate the temperature dependent elastic constants of h-BN. It has been noticed that the size of the h-BN sheet increases the thermal rippling, which not only decreases the thermal expansion coefficient and the elastic moduli, but also leads to a large deviation from the isotropic elasticity. The effect of changing the cut-off distance in the empirical potential on the stress-strain relation and the temperature dependent Young’s modulus of pristine and defective hexagonal boron nitride is also studied. The observed mechanical strength of h-BN is significantly affected by the vacancy and Stone-Wales type defects. The defect analysis shows that presence of vacancy type defects leads to a higher Young’s modulus, in the studied range with different percentage of defect concentration, in comparison with Stone-Wales defect. The estimation of various thermomechanical and elastic properties of h-BN has underlined their importance in many applications.